I. The major mutation among Ashkenazi Jewish Tay-Sachs carriers and patients is a 4 base pair(bp) insertion in exon 11 of the 14 exon HEXA gene resulting in a premature stop codon. HEXA alleles carrying the 4 bp insertion express mRNA that is rapidly degraded by an unexplained mechanism. We have developed a system to investigate the effect of the 4 bp on HEXA mRNA expression. A 8 kb "mini-HEXA gene" was constructed by fusing the coding region specified by exons 1 to 7 as a single block to a genomic segment containing the remaining exons and introns. This construct and derivatives were expressed in stably transfected L-cells and the HEXA mRNA levels were assessed by Northern analysis. We found that cells transfected with the normal construct contained readily detectable and properly spliced HEXA mRNA. Cells transfected with a mini-gene containing the 4 bp mutation, however, yielded undetectable HEXA mRNA. The mRNA level was restored with a mini-gene containing the 4 bp mutation when 1 bp was deleted upstream to correct the reading frame. II. Lysosomal enzyme genes exhibit an expression of lysosomal enzymes is enhanced. In order to gain insight into the regulation of the lysosomal enzyme genes we have characterized the promotor region of the HEXB gene. We found that both the mouse and human HEXB genes contain a homologous 50 bp region with functional promotor activity within 40 bp of the respective RNA start sites. The region contains several binding sites for transcription factors.